The present invention relates to a composition for foamed silicone rubber extrusions. Silicone rubber extrusions have long been used for a variety of applications, limited only by the imagination for the use of flexible plastic strips. Typical uses include baseboard molding strips or structural joining seals or strip sealants, but these are only illustrative of the range of available uses, particularly in light of the unlimited number of cross-sectional profiles which may be produced in the extrusion process by modifying the extrusion die. In the field of architecture, elastomeric architectural strips are used to cover joints between building sections, at roof lines and around windows, both in new construction and as a repair item for preexisting buildings. Silicone polymers are particularly important in the fabrication of architectural strips. These polymers are extremely flexible and strong, and their inertness makes them resistant to oxidation and photo degradation. Silicone rubbers may be fabricated in a variety of colors by the inclusion of pigments therein, so as to match a variety of substrates.
A wide variety of elastomeric articles can be prepared from silicones or other polymeric compositions by a process in which a resin composition is extruded to provide a shaped body which is subsequently cured to produce the finished item. In the context of the present disclosure, an extrusion process is defined to include any forming process wherein a precursor of the elastomer is forced under pressure into or through a shaping member such as a die or a mold to produce the shaped body. A number of shaping processes are known in the prior art. For example, U.S. Pat. No. 4,783,289 discloses a continuous extrusion process for the preparation of elongated silicone rubber members. According to the process, a hydrolyzable silicone composition is extruded from a die into a body of water to produce an elongated member, which passes through the body of water and cures while it is doing so.
The use of silicone based sealant fillers is similarly well known in the construction industry. These types of products are commercially available from Dow Corning Corp., Midland, Mich.: DC.RTM.732 Multi Purpose Sealant; DC.RTM.790 Building Sealant; DC.RTM.795 Building Sealant and from General Electric Company, Waterford, N.Y.; GE.RTM.1200; GE.RTM.SilProof. These products are used to fill in or seal or conceal joints in construction, such as expansion joints in structural wall or floor members. These sealants or caulks may be liquid (uncured) in storage but and if hydrolyzable, will rapidly begin to cure when exposed to typical atmospheric humidity. These sealants are typically stored in tubes and are dispensed through a nozzle at one end of the tube by forcing the enclosing wall at the opposite end of the tube toward the nozzle. This form of application is well known in the industry, and utilizes an applicator commonly referred to as a caulk gun. When the sealant flows from the nozzle of the storage tube it typically forms a bead of a diameter dependent upon the diameter and cross section of the nozzle and upon the rate of application.
An advantage of extruded silicone products is the relatively low cost of materials and processing, which in turn has created a great deal of competition and relatively low profit margins. Thus, there is a need in the art for extrudable silicone compositions which are more efficiently made at a lower cost. Many extrusion compositions require elevated temperatures to melt to an extrudable and mixable state and an elaborate or expensive curing process. To reduce costs, it is an advantage to be able to perform all steps of the extrusion process at or about room temperature. It has long been recognized that a blowing agent may be added to rubber compositions to create a foamed structure which is less dense. Such foamed compositions utilize less rubber per volume and thus, in certain applications, may have a lower material cost. However, many of the existing processes for creating an extrudable silicone rubber compound require special equipment for introducing the blowing agent, or require additional heating or mixing, or require special curing, or require expensive blowing agents or other components to produce a stable, flexible extrusion which may be readily processed and which has a reasonable shelf life and product life. In particular, most systems require temperatures greater than room temperature (approximately 20.degree. C.) for the reaction to activate the blowing agent, typically in excess of 25.degree. C. While such temperatures may not require much energy to generate within the mixing vessel, the equipment required to heat the mixing vessel can be costly. Thus, there is a need in the art for an inexpensive way to produce blown extrudable rubber composition. It is also a major concern in the use of blowing agents that the by-products or exhaust be inert so as to avoid potential health or environmental issues.
All extrusion compounds must be sufficiently viscous to be forced through the extrusion die, but then must cure into a sufficiently solid form for handling. The cure rate can greatly effect the physical properties of the resultant compound. Temperature is one variable that is typically controlled to govern the rate of cure, often through mechanisms as fans, heaters or water baths. With hydrolyzable compounds, the cure rate is effected by humidity and one method of speeding the curing process is to utilize a water bath. Unfortunately, the physical parameters such as temperature and humidity can vary widely, even within a single extrusion facility. Many extrusion lines exceed 200 feet in length, and it can be an expensive proposition to maintain the building containing the extrusion line at a constant temperature and humidity. Thus, the use of fans, heaters and/or water baths becomes a matter of constant adjustment and fine tuning. Variation in the rubber composition components can also affect the curing characteristics, but such variations also directly effect the chemical composition of the product. There is perceived to be a need in the art for a relatively low cost way to vary the cure rate for a silicone extrusion other than adjustment of the physical conditions during cure.
These and other advantages of the present invention will be readily apparent from the drawings, discussions and description which follow.